1. Field of the Invention
The present invention relates to an ignition control apparatus for an internal combustion engine which controls the length of time during which the primary current flows through the ignition coil.
2. Description of the Prior Art
An ignition coil dwell time control method for electronic spark advance systems of the type employing a microcomputer has been known in the art in which the desired dwell times as a function of the supply voltage and the engine speed are preliminarily stored in the form of a computing procedure or a map in the memory of the microcomputer, and a dwell time is determined by referring to it as occasion demands. Also, an ignition timing generating circuit including a microcomputer, counters, etc., has been known in the art in which a signal corresponding to a predetermined engine rotational angle and having a trailing edge indicative of an ignition timing is generated and an ignition coil dwell time is determined in accordance with the signal by a separate analog circuit.
In the case of the former method, the dwell times are predetermined, so that it is impossible to ensure the optimum dwell time due to the variations in coil performance caused during the manufacture, the coil temperature rise or the variations in spark plug discharge characteristics. Thus it is impossible to ensure satisfactory ignition performance. In the case of a circuit having a constant current control function for controlling the ignition coil primary current so as to prevent it from exceeding a predetermined value, if the duration of time that the constant current is reached is increased even slightly, the generation of heat by the circuit elements is increased rapidly, and this leads to breakdown of the elements in the worst case scenario.
While the latter method is free of the problems encountered by the former method, it requires a complicated circuit for controlling the dwell time and this increases the cost.